ENDANGERED SPECIES RESEARCHEndang Species Res

Vol. 5: 1–12, 2008doi: 10.3354/esr00103

Printed September 2008Published online July 1, 2008

INTRODUCTION

Once one of the most common cetacean species inthe Mediterranean Sea, the short-beaked common dol-phin Delphinus delphis has declined throughout theregion since the 1960s (Bearzi et al. 2003). In 2003, theMediterranean population was classified as Endan-gered in the International Union for Conservation ofNature (IUCN) Red List of Threatened Species, basedon observed declines in the number of animals andextent of occurrence, as well as deterioration in thequality of habitat in large portions of the Mediter-ranean. In 2005, Mediterranean common dolphinswere listed in Appendix I and II of the Convention onthe Conservation of Migratory Species (Bonn Conven-

tion, CMS). While it would be difficult to imagine amore favourable legal and international framework tosupport action in the region, conservation of commondolphins has remained ‘on paper’ (Bearzi 2007).

Although much of the Mediterranean basin has notbeen surveyed, there is clear evidence of populationdecline in portions where detailed studies have beenconducted, e.g. the northern Adriatic Sea (Bearzi et al.2004a) and the Gulf of Vera in southern Spain(Cañadas & Hammond 2007). Here, we document theprecipitous, ongoing decline of common dolphins ineastern Ionian Sea coastal waters across 13 yr. Also, weshow that the decline suggested previously for a corearea of 480 km2 between 1997 and 2004 (Bearzi et al.2005, 2006) has occurred over a wider area (1050 km2)

© Inter-Research 2008 · www.int-res.com*Email: giovanni.bearzi@gmail.com

Overfishing and the disappearance of short-beakedcommon dolphins from western Greece

Giovanni Bearzi1,*, Stefano Agazzi1, Joan Gonzalvo1, Marina Costa1,Silvia Bonizzoni1, Elena Politi1, Chiara Piroddi1, 2, Randall R. Reeves3

1Tethys Research Institute, Viale G.B. Gadio 2, 20121 Milano, Italy2Fisheries Centre, University of British Columbia, 2202 Main Mall, Vancouver, British Columbia V6T 1Z4, Canada

3Okapi Wildlife Associates, 27 Chandler Lane, Hudson, Québec J0P 1H0, Canada

ABSTRACT: Once one of the most common cetaceans in the Mediterranean Sea, the short-beakedcommon dolphin has declined throughout the region since the 1960s and in 2003 this population wasclassified as Endangered in the International Union for Conservation of Nature (IUCN) Red List.Here, we document the species’ precipitous decline in eastern Ionian Sea coastal waters across 13 yr.While 150 animals were present in the study area (1050 km2) in 1996, only 15 were observed in 2007.A 12 mo assessment of fishing effort and catch, together with circumstantial evidence, suggests thatthe decline was caused largely by prey depletion resulting from overfishing. We analyzed theimpacts of various fishing gear and estimated the degree of resource overlap between common dol-phins and local fisheries. The total biomass removed annually by 308 fishing boats in the study areaaveraged 3571 t, while that consumed by common dolphins was 17 t. Resource overlap between com-mon dolphins and fisheries — expressed as an average Pianka index of 0.5 — differed according tofishing gear, being higher for purse seiners (0.7) and beach seiners (0.4) and lower for bottom trawlers(0.1), trammel boats (0.2) and longliners (0.0). Only about 10 active purse seiners (4% of the totalactive fishing fleet) were responsible for 33% of the biomass removal, and likely had the greatestimpact on prey of common dolphins. This study indicates a high risk of local disappearance of com-mon dolphins in the very near future, unless fishery management measures are implemented imme-diately. Purse seining should be the main management target.

KEY WORDS: Short-beaked common dolphin · Fishing · Prey depletion · Mediterranean Sea

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OPENPEN ACCESSCCESS

Endang Species Res 5: 1–12, 2008

and a longer time interval (1995 to 2007). Bearzi et al.(2006) discussed possible reasons for the observeddeclines of common dolphins and other megafaunathat prey on epipelagic schooling fish and concludedthat the most likely cause of the declines was preydepletion as a result of fishing pressure. Here, we pro-vide quantitative data on fishing effort and landings inthe Kalamos area, and we link these data to the on-going local decline of common dolphins (and othermegafauna). Our 12 mo assessment is consistent withthe hypothesis that ecosystem damage has beencaused by unsustainable fishing (Stergiou et al. 1997,2007b).

Eastern Ionian Sea waters surrounding the island ofKalamos have been included by the Hellenic Ministry ofthe Environment in the Natura 2000 network (‘Sites ofCommunity Importance’) under the 9243 EEC ‘Habitats’Directive. In 2002 the Agreement on the Conservation ofCetaceans in the Black Sea, Mediterranean Sea and con-tiguous Atlantic area (ACCOBAMS), ratified byGreece, identified the area of Kalamos as one where pi-lot conservation and management actions should be de-veloped and implemented immediately to preserve thehabitat of common dolphins. In 2004, the ContractingParties to ACCOBAMS ‘welcomed’ aConservation Plan for Mediterraneancommon dolphins that identifiedKalamos as an Area of Conservation Im-portance (Bearzi et al. 2004b).

The study presented here (1) indi-cates that there is a high risk that com-mon dolphins will disappear from theKalamos area in the very near futureand (2) identifies fishery managementmeasures that must be implementedimmediately to prevent the furtherdecline of common dolphins and deteri-oration of the local ecosystem.

MATERIALS AND METHODS

Study area. The study area, situatedin eastern Ionian Sea coastal waters, in-cludes the Ionian islands of Meganisi,Kalamos and Kastos and covers 1050 km2

of sea surface (Fig. 1). The sea floor ismostly 50 to 250 m deep, with rockycoasts and shallows covered by seagrassmeadows. Waters are transparent (Sec-chi disk readings ranging between 10and 30 m) and oligotrophic (Gotsis-Skre-tas & Ignatiades 2007), and river andagricultural runoff is insignificant (Pittaet al. 1998).

Survey and photo-identification effort. Surveyswere conducted ad libitum from 4.7 to 5.8 m inflatablecraft with rigid hulls powered by 50 to 100 HP 4-strokeoutboard engines from 1995 to 2007. The survey cover-age totalled 34 801 km on effort and 908 survey days,from May to September each year (Table 1). Surveyswere conducted at speeds between 28 and 36 km h–1

under consistently good conditions: unimpaired visibil-ity, sea state ≤1 Douglas with no swell, and at least 2experienced observers scanning the sea surface (eyeelevation of approximately 2 m). Such conditions char-acterize ‘on effort’ in Table 1.

Photo-identification of individuals. Colour photo-graphs suitable for individual identification, based onlong-term natural marks on the dolphins’ dorsal fins(Würsig & Würsig 1977, Neumann et al. 2002), were ob-tained on 356 d. On each encounter with a group of dol-phins, as many good photographs as possible were takenof all individuals present. Photo-identification was per-formed following Würsig & Jefferson (1990), with cam-eras equipped with 70 to 200 mm f2.8 zoom lenses, using100 ISO colour transparency film from 1995 to 2002, anddigital photography from 2003 to 2007. Considering thatdigital photography substantially improves the effi-

2

Fig. 1. Study area (delimited by bold line) with bathymetric contour lines andlocations of fishing ports and other mooring/landing sites where local fishingfleets were monitored on a monthly basis from November 2006 to October 2007.The location of the study area relative to the eastern Mediterranean basin is

shown in the inset

Bearzi et al: Common dolphin disappearance in western Greece

ciency of individual dolphin identification (Markowitz etal. 2003), transparencies were scanned at high resolutionand turned into digital images for consistency of evalua-tion and analysis. Photos were then cropped around thedorsal fin and visible part of the body and selected usingconsistent criteria (i.e. entire dorsal fin visible, fin per-pendicular to camera, high sharpness and resolution, nowater spray masking fin profile), based on recommenda-tions provided by Read et al. (2003). Following such se-lection, the catalogue included 13 319 transparenciesand 4290 digital photos, totalling 17 609 dorsal fin pho-tos. These images were then matched, and the identifiedanimals were included in a database. Only individualswith distinctive dorsal fin profiles, bearing marks suit-able for reliable long-term identification from either sideof the fin, were used for mark-recapture analyses. Thecharacteristic white patch present on both sides of the finof some animals could be used to confirm a match (Neu-mann et al. 2002, Bearzi et al. 2005) but not as a distinc-tive feature. Individuals with a single tiny nick or no nickon the dorsal fin’s trailing edge, and individuals judgedto be recognizable only based on dorsal fin shape and/or

temporary scars were not used in theanalyses, as their inclusion could resultin borderline or incorrect matches andthus violate one or more mark-recap-ture assumptions. Patterns of site fi-delity implied by photo-identificationdata (Fig. 2, where information from1993 and1994 is also shown) and indi-vidual movements indicated by recor-dings of boat position (used as a proxyfor dolphin position) at 6 min inter-vals during photo-identification follows(Fig. 3), assisted in the selection ofthe most appropriate model for mark-recapture analyses.

Mark-recapture analyses. Mark-recapture methods rely on the numberof animals marked and their propor-tion in subsequent samples to esti-mate population parameters, includ-ing abundance (Seber 1982). Since the1980s these methods have been usedwidely to estimate abundance of cer-tain cetacean species based on photo-graphic records of naturally markedindividuals (Hammond 1990, Ham-mond et al. 1990, Wilson et al. 1999).Pollock’s robust design (Pollock 1982)with Huggins estimator has been thepreferred model. Previous work in theKalamos area showed that this popu-lation of common dolphins could beregarded as open among years and

closed within each annual sampling interval of 3 to 5mo (Table 1), based on a high degree of site fidelity,individual movement patterns, and low rates of immi-gration (Bearzi et al. 2005). The programme Mark 4.3(http://www.warnercnr.colostate.edu/~gwhite/mark/mark.htm) was used for mark-recapture abundanceestimation.

Monitoring fishing effort and landings. Prior to thisstudy it had been difficult to document fishing pressurein the study area due to (1) poor reliability of the land-ing data (Watson & Pauly 2001) — a well-known prob-lem in the wider Mediterranean and in the HellenicSeas in particular (Stergiou et al. 1997, 1998, Briand2000) — and (2) the fact that local fishermen oftendeliberately misreport their catches with the intentionof reducing taxation or avoiding stricter regulations(Bearzi et al. 2006). Estimation of biomass removal isessential to assess the impact of fisheries (Pitcher et al.2002). As reliable data on fishing effort and fisherylandings were unavailable, we incorporated in our dol-phin study a monitoring component aimed at obtainingdirect information on fishing within the study area.

3

Year May Jun Jul Aug Sep Subtotal

Daily surveys: common dolphin encounters1995 6: 3 13: 6 8: 4 27: 131996 3: 2 17: 15 22: 11 17: 6 59: 341997 15: 17 25: 18 20: 16 22: 14 22: 17 104: 821998 14: 11 17: 10 21: 18 20: 13 24: 17 96: 691999 16: 7 20: 15 22: 21 22: 8 80: 512000 22: 13 21: 12 24: 18 21: 11 88: 542001 1: 1 19: 16 18: 10 22: 7 17: 2 77: 362002 2: 0 17: 10 16: 3 23: 5 11: 1 69: 192003 10: 6 11: 3 18: 4 20: 5 59: 182004 4: 0 16: 2 11: 2 19: 3 11: 1 61: 82005 3: 1 15: 2 15: 3 17: 0 16: 1 66: 72006 1: 0 14: 5 14: 0 16: 0 18: 0 63: 52007 13: 1 14: 1 17: 0 15: 2 59: 4Total 40: 30 187: 92 204: 101 255: 102 222: 75 908: 400

Km surveyed ‘on effort’1995 225 566 378 11691996 35 413 850 477 17741997 231 608 543 432 576 23901998 313 367 623 625 597 25251999 541 510 716 689 24562000 867 504 901 723 29952001 1 465 541 962 500 24692002 50 574 501 1075 463 26632003 1137 860 1359 936 42922004 247 670 298 995 755 29652005 155 445 514 940 883 29372006 3 679 642 1079 918 33212007 566 887 949 444 2846Total 1000 6953 7061 11448 8339 34801

Table 1. Number of daily surveys, common dolphin encounters and km surveyed ‘on effort’

Endang Species Res 5: 1–12, 20084

Fig. 2. Pattern of site fidelity by year and month for 143 common dolphins photo-identified in the study area between 1993and 2007. Each row represents the sighting history of 1 individual

Fig. 3. Movements of com-mon dolphins in the studyarea between 1997 and 2007

Bearzi et al: Common dolphin disappearance in western Greece

In November 2006, we started a programme to mon-itor the local fishing fleet in all 16 ports and othermooring/landing sites (Fig. 1). All sites were monitoredonce per month on days of bad sea state conditionsand/or on Sundays, i.e. on days with a relatively highprobability that the whole fishing fleet would bemoored or in port. A total of 324 fishing boats were cat-alogued. Classification of fishing boats and gear wasbased on Nedelec & Prado (1990). The activity status ofeach boat was recorded visually every month based onfishing gear on board or near the boat’s mooring place,fishermen working on board, boat conditions, anddirect inquiries. The local fishing fleet — i.e. the boatsrecorded as active in any given month during the studyperiod — included 12 purse seiners (of which 11 were15 to 25 m and 1 was 12 m long), 24 beach seiners of8 to 12 m, 9 bottom trawlers of 20 to 25 m, 50 longliners(29 of 4 to 7 m and 21 of 7 to 10 m), and 213 trammelboats (98 of 4 to 7 m and 115 of 7 to 12 m). Boats shorterthan 4 m were not considered, as their impact wasassumed negligible. For the same reason a total of 16boats 4 to 7 m long equipped with gas-powered lampsand tridents/harpoons were also excluded from theanalyses. Some boats had multiple gears and switchedfrom one fishing method to another depending onseasonal closures (Kapantagakis 2007). For instance,most beach seiners operated as trammel boats duringmonths of beach seining closure. Therefore, activeboats were classified according to the gear used ineach month of sampling (e.g. a boat could be scored asa purse seiner in September and as a bottom trawler inOctober, depending on the gear used during thosemonths).

Landings of purse seiners, beach seiners and bottomtrawlers were monitored between November 2006 andOctober 2007. A total of 26 landings were recorded forpurse seiners (2.9 ± 1.9 mo–1, mean ± SD), 14 for beachseiners (2.3 ± 3.78 mo–1), and 16 for bottom trawlers(2.0 ± 1.31 mo–1). Total catch by species was recordedvisually by trained researchers based on the number offull boxes landed (boxes were routinely divided byspecies before landing). Full boxes were estimated toaverage 10 kg irrespective of species (an assumptionroutinely used for market purposes at landing sites,confirmed by direct observations). Days of activity permonth per boat for purse seiners, beach seiners andbottom trawlers were based on Kapantagakis et al.(2001), also taking into account periods of seasonalfishing closure. Average catch for trammel boats andlongliners was estimated as 3.42 t yr–1 for boats 4 to 7 mlong, and 6.31 t yr–1 for boats 8 to 11 m long (Stergiouet al. 2007c). Percentage catch contribution of the mostabundant species for longliners and trammel boatsoperating in the Ionian Sea was based on Stergiou etal. (2007a). Discard rates were assumed to be 14% for

purse seiners (Tsimenides et al. 1995 — a valuereported as likely to be an underestimate), 28% forbeach seiners (Stergiou et al. 1996), 39% for bottomtrawlers (Machias et al. 2001) and 9.8% for netters andlongliners (Stergiou et al. 2002, Tzanatos et al. 2007).Biomass removed annually by fisheries — calculatedseparately according to fishing gear—was the productof average daily catch per boat, days of activity permonth, number of boats recorded as active in eachmonth of sampling, and months of fishing activity.

Resource overlap. To assess the similarity of biomasscomposition between common dolphin prey and fish-ery catches, we used Pianka’s niche overlap index(a measure of resource overlap between 2 species;Pianka 1973, Pusineri et al. 2004):

where Pij is the percentage of prey item i of predator j,and Pik is the percentage of prey item i of predator k.The index ranges between 0 and 1, and the similarity ishigher the closer the index is to 1.

While the precise diet composition of common dol-phins in the study area is not known, diet preferenceswere inferred from (1) stomach contents of dead ani-mals in the study area (Bearzi 2000) and in neritic habi-tat elsewhere (reviewed in Bearzi et al. 2003, Cañadas2006), (2) observations of common dolphin feeding be-haviour and diving patterns in the study area (Bearzi etal. 2005), (3) direct observations of feeding in neriticMediterranean waters (Mussi & Miragliuolo 2003,Cañadas 2006), and (4) analysis of fish scales sampledduring surface feeding by common dolphins in thestudy area (Agazzi et al. 2004, Bearzi et al. 2006). Basedon this information, we assumed an average diet to becomposed of 80% Clupeidae and Engraulidae, 10%Belonidae, Gadidae and small Carangidae, 5% Lolig-inidae, and 5% other families (Table 2). Average dailyfood consumption was estimated as 4.2 kg animal–1 us-ing IB = 0.123M0.8 (Innes et al. 1987), where IB is the in-gested biomass (kg d–1) and M the body mass in kg.Body mass of common dolphins in the central Mediter-ranean averages 82.5 kg according to Cagnolaro et al.(1983). The body mass of juveniles was estimated as50% of adult mass; nursing calves were not considered.

RESULTS

Population trends

Mark-recapture estimates showed that common dol-phin numbers declined significantly and almoststeadily between 1995 and 2007 (Fig. 4). No mark-

α =×

( ) × ( )

∑

∑ ∑

P P

P P

ij iki

iji

iki

2 2

5

Endang Species Res 5: 1–12, 2008

recapture estimates could be obtained in 2006 (5 en-counters) and 2007 (4 encounters) due to insufficientrecapture events in different months. However, owingto the small group sizes and small number of encoun-ters, all animals sighted during those years werephoto-identified. There were 12 marked animals in2006 and 9 in 2007 (Fig. 4).

As mark-recapture estimates relied on natural mark-ings to identify individuals, they refer exclusively tothe population of marked animals. To include theunmarked portion and estimate total abundance, theproportion of unmarked individuals (which alsoincluded subadult classes) was computed based onthe number of photographs of marked and unmarkeddorsal fins obtained daily (Williams et al. 1993, Bearziet al. 2008). Based on 1858 high-quality dorsal finphotos taken in 1997 (the year with greatest photo-identification effort), the mean proportion of unmarkedanimals in the population was 0.44 (95% CI = 0.382 to0.494, n = 74). By adding this mean proportion of

unmarked individuals to the mark-recapture estimate,we obtained estimates of total population size (Fig. 4).

Even though mark-recapture analyses were pre-cluded in 2006 and 2007 by insufficient recaptures inthose years, encounters were so few (Table 1), groupsso small (range 2 to 9), and photo-identification effortso extensive that we had confidence in our ability tocount and photograph all individuals present duringthe sightings. Direct counts of total animals photo-graphed (18 individuals in 2006, 15 in 2007) were con-sistent with values obtained by adding the estimatedunmarked animals to the number of marked animals(21 individuals in 2006, 16 in 2007), but field countsappeared to be the most accurate and were thereforeused to complement information obtained throughmark-recapture (Fig. 4).

The observed patterns of site fidelity (Fig. 2) anddolphin movement (Fig. 3) suggest that the declineindicated by the mark-recapture analysis applies to theentire local population of common dolphins.

Impact of fishing and resource overlap

Landings by purse seiners averaged 706.2 kg (SD =967.91, n = 26, range 0 to 4030). Species landed werepredominantly horse mackerel Trachurus trachurus(26.2% of the total catch), European pilchard Sardinapilchardus (23.1%), bogue Boops boops (15.8%), Euro-pean mackerel Scomber scombrus (11.2%), round sar-dinella Sardinella aurita (10.6%; sometimes discardedbefore or during landing operations due to their lowmarket value), picarel Spicara smaris (7.5%), Atlanticbonito Sarda sarda (1.9%), European squid Loligo vul-garis (1.7%), European barracuda Sphyraena sphy-raena (1.0%) and skipjack tuna Katsuwonus pelamis(0.8%). Landings by beach seiners averaged 125.6 kg(SD = 101.55, n = 14, range 29 to 410). Species landedwere predominantly picarel (56.1%), European pil-chard (26.5%), bogue (6.83%), European squid(3.58%), European barracuda (1.8%), striped red mul-let Mullus surmuletus (1.3%), greater amberjack Seri-ola dumerili (0.9%), European mackerel (0.6%) andannular seabream Diplodus annularis (0.6%). Other

6

Beach Purse Bottom Trammel Longliners Commonseiners seiners trawlers dolphins

Clupeidae, Engraulidae 26 34 0 13 0 80Belonidae, Gadidae, small Carangidae 0 26 17 0 0 10Loliginidae 4 2 15 0 0 5Large Carangidae, all other Families 70 38 68 87 100 5

Table 2. Estimated percentage composition of catches by fishing gear and inferred diet composition of local common dolphins(see ‘Resource overlap’ section in ‘Materials and methods’)

Fig. 4. Number of common dolphins in the study areabetween 1995 and 2007, estimated through photographicmark-recapture. h: total number of marked animals photo-identified; j: mark-recapture estimates; d: total population

estimates (marked and unmarked animals)

Bearzi et al: Common dolphin disappearance in western Greece

species caught in small quantities (0.4 to 0.1%) in-cluded gilthead seabream Sparus aurata, Octopus spp.,salema Sarpa salpa, saddled seabream Oblada mela-nura, common pandora Pagellus erythrinus, Europeanseabass Dicentrarchus labrax and common cuttlefishSepia officinalis. Landings by bottom trawlers averaged495.3 kg (SD = 350.40, n = 16, range 195 to 1510). Spe-cies landed included predominantly European hakeMerluccius merluccius (36.6%), horse mackerel(17.0%), European squid (14.9%), red shrimp Aristeusantennatus (11.4%), striped red mullet (7.7%), bogue(4.1%), angler Lophius piscatorius (1.7%), thornbackray Raja clavata (1.4%), common pandora (1.1%), silverscabbardfish Lepidopus caudatus (1.0%), spottail man-tis shrimp Squilla mantis (0.6%), European seabass(0.5%), and Octopus spp. (0.5%). Other speciescaught in small quantities (0.4 to 0.1%) included pi-carel, small-spotted catshark Scyliorhinus canicula,common cuttlefish, gilthead seabream, flathead mulletMugil cephalus, and common sole Solea solea.

Estimates of the total biomass removed by the localfishing fleet between November 2006 and October2007 and a comparison of the relative impact of differ-ent fishing boats are shown in Table 3, taking intoaccount days of inactivity and discards (see ‘Materialsand methods’). The estimated total biomass removedby local fisheries was 3571 t. A Pianka’s index of 0.5reflects the likely present overall degree of resourceoverlap between common dolphins and fisheries in thestudy area (Table 2). Resource overlap differed accord-ing to fishing gear, being 0.7 for purse seiners, 0.4 forbeach seiners, 0.1 for bottom trawlers, 0.2 for trammel

boats, and 0.0 for longliners (Fig. 5). The apparent lackof resource overlap between common dolphins andlongliners is consistent with the catch compositionobserved in the Cyclades Islands (Stergiou et al. 2002)and in the Patraikos Gulf (Tzanatos et al. 2006). Thebiomass consumed by 15 common dolphins estimatedto be present in the study area during the same timeinterval was about 17 t (Fig. 5).

DISCUSSION

Once common and relatively abundant in the area ofKalamos, common dolphins have declined dramati-cally over the past decade. While approximately 150animals used the study area in 1996, only 15 were ob-served in 2007. Tuna and swordfish also have declined(Bearzi et al. 2006). Large tuna, in particular, werecommonly encountered in the early years of the studybut were rarely seen after 2001. The decline of high-order marine predators that feed on epipelagic prey isconsistent with the hypothesis that intensive exploita-tion of epipelagic fish stocks reduced the availability ofkey prey, making this coastal habitat less able to sus-tain populations of large predators (Bearzi et al. 2006).Other factors that might have played a significant roleinclude unrecognized oceanic changes, tissue contam-ination by xenobiotic chemicals and incidental mortal-ity in fishing gear, but only the last is likely to havebeen relevant. A local worker reported that 5 commondolphins with mutilations were found stranded nearPaleros between December 2004 and April 2005. Of

those, only 1 animal could be exam-ined and based on mutilation of its tailflukes, it likely died in fishing gear(Bearzi 2006). No other reports werereceived of common dolphin bycatchduring the entire 15 yr study (1993 to2007) or during the 12 mo of fisherymonitoring. Also, floating carcasses ofcommon dolphins (possibly indicativeof bycatch) were never observed overa total of 1163 d spent at sea withinand around the study area since 1991,totalling >67 000 km of navigation. By-catch is an important cause of mortal-ity of common dolphins in some partsof the world (Tregenza et al. 1997, Tre-genza & Collet 1998, Julian & Beeson1998) and it may have contributed tothe observed decline in common dol-phins around Kalamos. However, weconsider it unlikely that bycatch wasthe sole or even the main cause of thedecline.

7

Boat kind Boat length Mean no. Active Annual % of total(m) of active months biomass annual

boats removal (t) biomassremoval

Purse seiner 15–25 7.5 9.5 1166.5 32.71.0 6

12 1.0 8Beach seiner 8–12 1.0 4 528.7 14.8

23.0 6Bottom trawler 20–25 6.0 8 797.8 22.3

1.0 6Trammel 4–5 22.8 12 915.4 25.6

5–7 49.4 127–10 15.5 6

83.9 1210–12 1.3 6

2.2 12Longliner <7 15.3 12 162.7 4.6

7–9 10.9 129–10 4.5 12

Table 3. Composition of the fishing fleet operating in the study area and biomassremoved annually

Endang Species Res 5: 1–12, 2008

As an alternative hypothesis, it is necessary to con-sider the possibility that the observed trends were aconsequence of emigration or long-range movements.However, surveys outside the study area have pro-vided no evidence to support the suggestion that com-mon dolphins have shifted their distribution away fromthe Kalamos area (Bearzi et al. 2005). This area hasbeen one of the few remaining pockets of high-densityoccurrence in the central and eastern Mediterranean,and there are few reports of common dolphins in sur-rounding areas. Dedicated cetacean surveys con-ducted by the Tethys Research Institute in the IonianSea between 1991 and 2007, covering more than8200 km outside the Kalamos study area, produced nosightings of common dolphins. Notwithstanding sub-stantial dedicated effort by several research groups,only 3 sightings have been reported to date from east-ern Ionian Sea waters outside the study area (1 in eachof 1992, 1995 and 1998; Angelici & Marini 1992, Politiet al. 1992, Frantzis et al. 2003, Gannier 2005). Be-tween 1999 and 2007, common dolphins were neverseen north, west or south of the study area duringcetacean surveys totalling tens of thousands of kilo-metres of dedicated effort and covering large portionsof the eastern Ionian Sea, as well as waters west of thePeloponnese (Frantzis et al. 2003, Lacey et al. 2005,IFAW 2007, A. Frantzis pers. comm.). More generally,common dolphins appear to be rare in, or absent from,the adjacent central Mediterranean areas explored sofar, including the Hellenic Trench, other neritic andpelagic portions of the Ionian Sea, and the Adriatic Sea

(Notarbartolo di Sciara et al. 1993, Bearzi et al. 2003,Frantzis et al. 2003, Gannier 2005, Lacey et al. 2005,IFAW 2007), the only exception being the inner Gulf ofCorinth, where a few common dolphins remain(Frantzis & Herzing 2002). Recent genetic evidence(Natoli et al. 2008) indicates a relatively high degree ofdifferentiation among common dolphins in the Medi-terranean and shows that common dolphins in theeastern part of the basin, including the Ionian Sea, arelargely isolated from the western Mediterraneanpopulation.

Prey depletion caused by overfishing is thought to beone of the main reasons behind the overall decline ofMediterranean common dolphins (Bearzi et al. 2003,2004a, Cañadas & Hammond 2007). The declines infish stocks and loss of marine biodiversity caused byfishing pressure are a growing concern worldwide(Pauly et al. 1998, 2002, Jackson et al. 2001, Pitcher2001, Csirke 2005, Worm et al. 2006). Unsustainablefishing has been implicated in dramatic ecologicalchanges in the Mediterranean Sea (Sala 2004), whereit has caused the decline of many fish stocks (Caddy &Griffiths 1990, De Walle et al. 1993, Stanners & Bour-deau 1995, Caddy 1997, Garcia et al. 2005). Some ofthe Mediterranean fish stocks that have been over-exploited include important prey species of commondolphins (Lleonart 2005).

While the impact of fishing remains to be assessed atthe regional scale, there is ample and growing evi-dence of overfishing in the Hellenic Seas, includingthe eastern Ionian Sea. Around the island of Kalamos,

8

Fig. 5. Estimate of total biomass removed by common dolphins and fisheries in the last year of the study. The darker part of thefisheries bars shows removal of species representing key prey for common dolphins, as suggested by the index of resource overlap

Bearzi et al: Common dolphin disappearance in western Greece

total landings have decreased since the mid 1980s(Papaconstantinou et al. 1985, 1988, Papaconstantinou& Stergiou 1995, Stergiou et al. 1997). European Com-mission (EC) figures indicate that catch per day of bothdemersal and pelagic resources has declined steeply inthe Hellenic Ionian Sea over the last decade (EC 2004).In the eastern Ionian Sea, in particular, purse seinecatch per day of both anchovies and sardines declinedsince the mid 1990s (EC 2004), especially for vesselslarger than 15 m, which account for the majority of thecatch of small pelagic fish species (Stergiou et al.2007b). The long-term increasing trends in Hellenicmarine landings from 1964 to 1994, attributed to fleetmodernization and geographic expansion of the fish-eries over this period, have been followed since themid 1990s by rapidly declining trends in landings andyields, suggesting that the fishing has been overlyintense, i.e. unsustainable (Stergiou et al. 1997, 2007b,Stergiou 2005).

Overlap between dolphin prey species and fisherytarget species — as indicated by the Pianka’s index —does not prove direct competition (Briand 2004). How-ever, it is reasonable to infer competitive effects whenkey prey becomes scarce and remains subject to heavyfishing pressure (Trites et al. 1997, Kaschner & Pauly2004, Pusineri et al. 2004). Around Kalamos, the poten-tial for exploitative competition (Keddy 1989) betweenhigh-order predators such as common dolphins andlocal fisheries targeting their prey is apparent (Fig. 5).Overfishing and destructive fishing methods affect notonly top predators, but also the fishery and in turn thefishing communities. Though it was not possible toquantify this aspect as part of the present study (cf.Saenz-Arroyo et al. 2005), local artisanal fishermenconsistently lamented the declines in catches and re-ported to our research team that the local marine eco-system had become less productive in recent decades.

The waters around Kalamos are an importantspawning area for European pilchard (Somarakis et al.2000, 2006a,b, Machias et al. 2007) and hake (Politouet al. 2006, Politou 2007), making this Natura 2000 Siteof Community Importance a candidate for special pro-tection based on EC Regulations for the sustainableexploitation of fishery resources in the Mediterranean(EC 2006). In addition to common dolphins, the area ishome to species included in Annex II to the HabitatsDirective, including a resident community of commonbottlenose dolphins Tursiops truncatus and endan-gered species such as the Mediterranean monk sealMonachus monachus and the loggerhead sea turtleCaretta caretta.

In view of the evidence provided here, and consider-ing the existing political and legal commitments toconserve cetaceans and preserve marine biodiversity(see ‘Introduction’) (Owen 2004), relevant local, na-

tional and regional governmental bodies are obliged totake action to reduce fishing pressure and limit the useof fishing gears that can have unintended harmful ef-fects on the marine environment. Fishery managementmeasures are urgently needed to reduce current over-exploitation and allow for the recovery of endangeredmarine megafauna. Such measures should include thefollowing: (1) an immediate moratorium on purse sein-ing, as advocated by the large local community of arti-sanal fishermen; (2) restrictions on bottom trawling;(3) strict enforcement and appropriate penalties for il-legal fishing; and (4) full implementation of the Coun-cil Regulation 1967/2006, which also demands a banon beach seining by May 31, 2010 (beach seining isconsidered harmful to fish stocks and habitats and hasbeen banned in most EU Countries). As EC fundingtools exist to compensate the affected fisheries, thismay be a ‘win-win’ situation, where existing regula-tions can be effectively implemented to solve a specificconservation problem, with the added value of protect-ing marine biodiversity, ensuring continued ecosystemservices, preserving artisanal fisheries, and bringinglong-term benefits to human society.

Acknowledgements. This work was supported in part by aPew Marine Conservation Fellowship (a programme of thePew Institute for Ocean Science), by OceanCare, by TheWhale and Dolphin Conservation Society, by the UNEP/CMSAgreement on the Conservation of Cetaceans of the BlackSea, Mediterranean Sea and Contiguous Atlantic Area, andby UNEP’s Regional Activity Centre for Specially ProtectedAreas. Thanks to S. Bruno, S. Ferretti, A. Frantzis, A. Natoli,A. Petroselli and many other collaborators for contributing tofield data collection and data analysis. Our gratitude also goesto all the volunteers who made this research possible throughtheir financial support and help in the field. A. Frantzis, T.Lewis, D. Moutopoulos, G. Paximadis, E. Tryfon and A. I.Tsikliras contributed relevant information. Constructive com-ments were provided by C. M. Fortuna, G. Notarbartolo diSciara, D. Pauly and 2 anonymous reviewers. R. Gramolinideveloped a dedicated tool to ease ArcView GIS analyses.The Milan Civic Aquarium and Hydrobiological Station pro-vided logistical support. The work presented here was con-ducted under research permits issued by the Hellenic Min-istry of Agriculture, General Directorate of Development &Protection of Forests and Natural Environment, Directorate ofAesthetic Forests, Woodland and Hunting.

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Editorial responsibility: Rebecca Lewison,San Diego, California, USA

Submitted: January 2, 2008; Accepted: May 1, 2008Proofs received from author(s): June 25, 2008